Micro Electro Mechanical Systems (MEMS) processing is being investigated as an alternative for fabricating extreme ultraviolet (EUV) transition radiation laser (TRL) and transition radiation oscillator (TRO) structures. MEMS TRO fabrication can involve forming layers of membranes spaced apart by layers of intervening material or vacuum regions. The total number of membranes, membrane thickness, membrane spacing, and presence or absence of the intervening material can all affect the output wavelength of the TRO device. When fabricating TRO devices that operate at higher frequencies, for example in the extreme ultraviolet (EUV) region (around 13 nanometers), it may be necessary to remove intervening material between membrane layers to insure proper TRO device operation. Removal of intervening material can produce problems with TRO device operation if it damages the membranes or results in membrane shape changes.
Shown in FIG. 1A, is a cross section of a prior art EUV TRL (or TRO) structure 100. Under normal operation, relativistic particle energy 140 is directed through membranes 120 to produce EUV radiation 150 at the output of the structure 100. In this example, intervening material has been removed from between membranes 120 thereby forming vacuum regions 130. The structure 100 thus comprises alternating layers of membrane regions 120 and vacuum regions 130. The edge of membrane regions 120 are held in place by support posts 110. Here, even though membranes 120 are not supported or spaced apart by an intervening material layer, the membranes' surfaces 132 are of uniform shape and planer relative to each other. The distance 134 between membranes is relatively constant along the surface 132 of the membrane. In addition, this distance is also relatively constant from membrane-to-membrane. Under these conditions, the structure 100 operates normally to produce a coherent superposition of transition radiation at EUV wavelengths.
Shown in FIG. 1B, is a TRL (or TRO) structure similar to that shown in FIG. 1A. However, In FIG. 1B, some membranes 121, 123 have become defective or experienced shape changes. These membrane defects/shape changes can include membrane bowing 122, membrane breaking 126, and membrane support post separation/fracture 124. Membrane bowing 122 can be the result of thermally induced stress resulting from the high temperature operation of the TRL (or TRO) device. Membrane breaking 126 can result from mechanical or thermal stress induced into the membrane 125 during operation, handling, or fabrication of the structure. Membrane support post separation 124 can result when the membrane's intrinsic stress exceeds the fracture point of the interface between the support post 110 and the membrane 130. When this occurs, the membrane 123 can snap away from the support post 110, as shown in FIG. 1B.
The membrane shape changes can impact the structure's performance by producing aberrational output radiation 160. The effect of this aberrational output radiation 160 can be manifested in a variety of undesirable ways that include shifts in the output radiation's peak intensity or wavelength.